Finland. the Oripää esker

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1 VESIENTUTKIMUSLAITOKSEN JULKAISUJA PUBLICATIONS OF THE WATER RESEARCH INSTITUTE Veli Hyvärinen: River discharge Tiivistelmä: Virtaamaolot Suomessa Reijo Solantie in Finland & Matti Ekholm: Water balance in compared to Tiivistelmä: Suomen vesitase Oleg Zaitsoff: Groundwater balance Tiivistelmä: Oripään harjun pohjavesitase Finland during the period as verrattuna vuosien vesitaseeseen 24 in 54 the Oripää esker 3 VESIHALLITUS NATIONAL BOARD OF WATERS, FINLAND Helsinki 1985

2 Tekijät ovat vastuussa julkaisun sisällöstä, eikä siihen voida vedota vesihallituksen virallisena kannanottona. The authors are responsible for the contents of the publication. It may not be referred to as the official view or policy of the National Board of Waters. 1S B N ISSN Helsinki Valtion painatuskeskus

3 24 WATER BALANCE IN FINLAND DURING THE PERIOD AS COMPARED TO Reijo Solantie & Matti Ekholm SOLANTIE, R. & EKHOLM, M Water balance in Finland during the period as compared to Publications of the Water Research Institute, National Board of Waters, Finland, No. 59. Runoff, precipitation and evaporation were determined as annual means for the period , both basinwise and by isoline analysis on maps. Precipitation was corrected for measuring errors. Evaporation was calculated in two ways, firstly as the residue from the water balance equation and secondly by an equation rendering evaporation from land areas as a function of three parameters representing air temperature, the amount of growing stock and the effect of open bogs. Further, the changes of these main components of the water balanee between the periods and were investigated. Index words: Water balance, corrected precipitation, evaporation estimation, runoff. 1. INTRODUCTION Hydrological and climatological statistics are in the first place calculated for normal periods, successive periods of 30 years, according to WMO practice. At present (1984) the period is in progress. However, some in formation concerning the components of water balance since the end of the last 30year period is aiready needed in the nineteeneighties. To obtain this information, it is practical to use the halves of the normal periods. Results for 10year periods are somewhat inaccuate mainly due to storing problems, and 20year periods cannot be added to form normal periods. The period was chosen also because the main com ponents of the water balance are climatological parameters or depend on these, and the most recent climatological statistics are available for the period (Heino 1976). ln this study the annual means of the main components of the water balance were calculated, both basinwise and as map analyses. 2. BASINS USED IN THE STUDY Areal values were calculated for precipitation (P), runoff (R) and evaporation (E). Evaporation was caiculated both frorn the water balance equation (EB) and from the evaporation equation devel oped by Solantie, Equation 2, section 6.1. For obtaining EB, the change in water storage per

4 25 ob year (M) was also needed. Only basins with servations available for the whole period were used. For some of the basins, the changes in the components from the period to the period could be obtained. The areal runoff and corrected precipitation were calcuiated for the basins on the basis of data from discharge observation sites (National Board of Waters 1980). Ali the basins were cluded for which EB could be obtained accurately enough as the difference between P and R+M. In addition, the basins Peerajärvi, Kilpisjärvi and Utsjoki were included. For these basins P was tained as the sum of R and Ek because orographic effects and exposed terrains make observed precipitation inaccurate. The basins are listed in Appendix 1 and shown in Fig. 1 lb). Those basins for which the standard error of Enoted by S (EB) and obtained (P)+S2 (R), was greater than 25 mm,were rejected. However, the aforementioned three basins in northern Lapland were retained. Furthermore, in regions with particular features in the water balance, values of S(EB) up to 30 mm were accepted if more curate results were not available. The regions are as follows: 1. Regions including basins bounded by flow observation sites both downstream and stream called in this paper intermediate basins. In these basins values of R are tained as residues, being therefore somewhat Iess accurate. 2. Regions of areal precipitation maxima, mostly including small basins around river headwaters or on the coasts. 3. Northern Lapiand, where precipitation stations are far apart. Because runoff is determined more accurately than precipitation, S(R) was negiected. S(P) = 25 mm eorresponds to a certain minimum of the area of the basin, denoted by Fmjn. Further, Fmjn depends on the density of precipitation stations Because p was in 1982 approximately the same as in the period , it was cuiated by latitudinal zones from the station iist for 1982 of the Finnish Meteorological Institute by rnultiplying the latitudinal numbers of the stations by 0.8 which is the proportion of cepted stations (see Section 4.1). Minimum areas are shown in Tabie 1. For obtaining EB, 17 too small basins were accepted (2 at the southern coast, 3 at the tauon maxima on the Suomenseikä divide, 9 in (p). 2 as.js (la, in ob ac up ob cal ac precipi Table 1. Density of precipitation stations (p = number of station per and the minimum area of the basins (Fmin) by latitudinal zones. Zone Latitude degrees () km2) P km < <» > < the watershed regions of the provinces Kainuu and Koillismaa and 3 in northern Lapiand). In the intermediate basins, (29 % of ali, denoted by index v) P., becomes less accurate with smaller vaiues of the parameter VF: S(Pv) S(P) VF /1+(1 vf2 (1) where standard error of P in intermediate basins 8(P) = standard error P in other basins VF = proportion of the intermediate basin (bounded by discharge observation sites both downstream and upstream) of the totai basin area upstream of the stream site km2 km2) km2, down The 30 mm upper iimit of S(EB) determines a iower iimit for vf, decreasing with decreasing 8(P) or with increasing p and F. Most intermediate basins occur in the iake district of Finland, represented by zone b2. Cor responding to the value of p for zone one obtains 1/4 as the lower limit of vf for iarge basins (about and 1/3 as that for small basins (about km2). By dividing basins into parts down to these accuracy iimits of area or of vf, 72 separate basins were obtained, for which the water balance components couid be determined with mutual independence. However, in some cases a very sma[l basin is within a very iarge one. The principie of mutual independence of basins was in such cases only slightly injured, because the sizes of the two basins were of different orders. The mean area of these 72 basins was and the total area or 74% of the area of Finland. In addition to these 72 basins, the water balance components were obtained for 16 binations of these basins. Regarding further the 3 before mentioned cases in Lapland, the water com F

5 Fig. la. Basins for which the water balance was determined for the period

6 27 Fig. lb. Largebasins for which the water balancewas determinedfor the period in addition to the balance of their subbasins. = determined only separately > = determined separately and as part of a larger basin = determined separately and as part of two larger basins

7 28 balance components are given for 91 basins in total. Of the mutually independent basins, the changes in water balance components from period to period could be obtained for 39 basins. 3. RUNOFF 3.1 Basic data The basic data for the mean annual runoff (R) during the period comprises meas urements made by the Hydrological Office of the National Board of Waters, Finland at flow gauging and hydroelectric power stations. Large basins were divided into subbasins by using discharge series at sites inside them, which were above the size limit below which precipitation is too inaccurate (Section 2). If only a few years of observations were lacking these were inter polated by using reference values from neigh bouring stations. 3.2 Location and correction of erraneous values Some observation series, for which R disagreed with corresponding values for neighbouring stations and other water balance components, were subjected to a closer examination. The error and the correct value could be calculated by using values of R observed at neighbouring stations and estimates of R from the water balance equation for the remainder of the basins. However, because neighbouring stations were available, the estimated values were not used in this study. Only in one case (Kilpisjärvi basin) was the value of R really needed. The values of P around Kilpisjärvi basin were inaccurate and only one neighbouring dis charge series was available. Therefore, the dis charge curve was examined closer. It was found to be partly erraneous and corrected for obtaining the required value of R. The corrections made were as follows: 1. Discharges at Kilpisjärvi (Q 819), Iocated in the upper part of the Tornionjoki basin, were reduced by 13 % after an examination of the discharge curve in the nineteenseventies. Con sequently, the corrected value of R (490 mm) was used instead of the original (565 mm) for the Kilpisjärvi basin (denoted by 81 in Fig. 1 and Appendix 2). The resuit is somewhat uncertain because the time when the discharge curve changed is unknown. However, the corrected value is rather close to the cor responding value for the neighbouring Peera järvi basin (449 mm), located at about the same altitude. 2. The corrected value of R for the lijoki basiji upstream of Raasakka hydroelectric power station (419 mm) is 16 mm greater than the original value (403 mm), obviously because of problems caused by change of observation site during the period. Consequently, the dis charge series here was replaced by two others in combination, namely those for the Kipinä flow gauging station and the Leuvankoski flow gauging station at the Siuruanjoki tributary (the lijoki basin is denoted by 61 in Fig. 1 and Appendix 2). 3. The corrected value of R for the basin up stream of Kaukonen (Q 846) on the river Ounasjoki is 331 mm or 7 % smaller than the corresponding original value of 356 mm. The discharge series was replaced by that at Köngäs (Q 840, the basin upstream denoted in Fig. 1 and Appendix 2 by 73). 4. The corrected value of R for the upper part of the Kymijoki basin above the Vaajakoski hydroelectric power station (300 mm) is about 8 % greater than the corresponding original value of 278 mm. This discharge series was replaced by two others in combination, one for Simunankoski (Q 31, the basin upstream denoted by 17 in Fig. 1 and Appendix 2) and the other for Kapeekoski (Q 825, the other upstream basin, denoted by 23). 5. The corrected value of R for the Kymijoki basin above the Kuusankoski hydroelectric power station (279 mm) is 7 % larger than the corresponding original value of 261 mm. This discharge series was replaced by two other series in combination, the one for Kalkkinen (Q 55, the next basin upstream, denoted in Fig. 1 and Appendix 2 by 14) and that for Ripatinkoski (Q 63b, the tributary upstream basin denoted by 10). 3.3 Laying out of the runoff maps The mean annual runoff (R) duringperiod is also shown cartographically (Fig. 2). For the details of the isolines, maps ofp (Fig. 3) and E

8 S 29 Fig. 2. Meafl annual runoff during the period (mm). Fig. 3. Mean annual precipitation (corrected) during the period (mm). (Fig. 4) were used. The change in R from period to period is shown in Fig Temporal variation in the annual runoff The temporal variation in the annual amounts of runoff (R0), precipitation (P0) and evaporation was studied in a sample of eight basins (the basic material: Hydrografinen toimisto 1935, 1936, 1938, 1944, 1948, Tie ja vesirakennus hallitus 1954, 1957, 1962, 1963, 1965, 1968, 1970, National Board ofwaters 1972, 1975, 1976, 1977). These basins are not included in the list of basins for which the water balance was deter mined (Appendix 2). However, m Appendix 2, basins or their combinations practically the same as those used here may be found. There are two (E0) reasons for such a choice of the basins. Firstly, basins as large as possible are needed for avoiding the disturbances caused by changes in the station sets, particularly in northern Finland. Secondly, accurate values of snow storage are needed for calculating annual values of evaporation (Section 6.7). The standard deviation of the annual amounts of runoff, denoted by S (R0), was cal culated both for period and period The change of S (R0) between the periods, denoted by d S (R0), was also obtained. Further, the smoothing of the variation of R0 due to water storage was studied in the light of the dif ferences S (R0) (P0). Because of the short ness of the periods, these differences were cal culated for the period These results and some information concerning the basins are presented in Table 2. The values of S (R0) were in ail basins ome

9 what smaller in period than in period , these changes being however not statistically significant at the 5 % level. According to the statistics for the period , S(R 0) is markedly smaller than S (P0) in the basins where L is about 20 %, as is usual in the Lake district. Thus, storage in large lakes for periods longer than one year markedly smoothes the variation in runoff. This effect is somewhat weaker in the basin Kokemäenjoki, Harjavalta, belonging only partly to the Lake district. The storage of water in aapa bogs is reflected in the rather small values of S (R0) in comparison to S (P0) in the two last basins. Elsewhere in Finland, excluding the regions of aapa bog complexes and the Lake district, S (R0) is greater than S (P0). Fig. 4. Mean annual evapotranspiration from land and water areas during the period (mm). 4. PRECIPITATION 4.1 Correction of measuring errors in pre cipitation for obtaining mean annual precipitation during the period basinwise and as isolines on a map The basin values (Appendix 2) and a map of cor rected values were obtained by correcting the map and basin values of mean annual precipitation (National Board of Waters 1980) for errors in Table 2. The standard deviation of annual values of R0(denoted by S (R0) in periods and , its change between the periods (denoted by d S (R0), and its deviation from S (P0). Additional information concerning the basins is also given. Basin Corresp. basin S (R0) mm ds (R0) S (R0) S (P0) mm in Appendix 1 mm andfig. la ( )+( ) Vuoksi, Imatra Kymijoki, Kalkkinen Vantaajoki,Oulunkylä Kokemäenjoki, Harjavalta Kyrönjoki, Lansorsund Kalajoki, Hihnalankoski 1) lijoki, Merikoski Kemijoki, Isohaara Average ) During the period Kalajoki, Hihnalankoski (F = km2, L = 1.8 %). During the period Kalajoki, Niskakoski (F = km2, L = 1.8 %).

10 31 measurement. For the preparation of this map of observed precipitation, both basin and station specific values had been used. Each basin value had been obtained as an average of the 180 monthly values of period Con sequentiy, it had also been possible to use short observation periods. In each month, values from about 120 stations had been rejected and about 500 accepted for calculating basin values. Because basins with calcuiated values cover most of Finland, long period averages from single stations (278) had mostly been used only for a more close analysis within large basins. The correction of annual precipitation due to the rneasuring error (about 100 mm) consists of the adhesion correction and windplusevap oration correction. The long period average of the annual adhesion correction (T) is about 26 mm (Soiantie 1976) and that of the wind plusevaporation (B) about 74 mm. Because the relative windplusevaporation correction (per cent of observed precipitation) varies depending on the forms of precipitation being at its greatest during dry snowfalls, the long period average of its mean value depends on the region as well as on the proportions of different forms of precipi tation. In addition, it depends on the exposure of the observation site (a) being proportional to the exposure number of the site (Korhonen 1941). Values of c are calculated as weighted means of its components in different compass directions (oj), the weights being the frequencies of winds during precipitation. Values of rj vary between 0 (a totally shielded sector) and 1 (a totaily exposed sector). The mean of a,, iso. 35 and its standard deviation S (a) about Consequently, S(B) 38 mm. The number of stations in a typical large basin (8 000 km2) is about 15 and in a typical small one (1 200 km2) about 3 (because stations outside the basin but ciose to its boundaries are also inciuded). Now it is possible to correct the basin values of P so that the exposure at ali stationsis approxi mated by.for such a correction (B) the stan dard error S(B) due to the approximation amounts for large basins to about 10 mm and for smali basins to about mm. These vaiues are the upper iimits of errors, however, because the most exposed stations have been discarded. The parameters from the accepteds1tations are denoted by the index C. In the qualified material, b and S(B aresma1ler than, 5 and S(B) in the totai material. Since S(B) <S(B), values of were regarded as sufficiently accurate, i.e. wind piusevaporation correction was determined at ali stations as a function of F. A map analysis of the relative total correction of mean annual amounts of precipitation based on the total material was made for period These relative corrections (denoted by k) can also be applied to the corresponding material of period if the proportions of dif ferent forms as weli as the monthly amounts of precipitation are equal during both periods. However, the application of k to the accepted material of period ieads to an over correction (biased correction). The ievel of bias was determined using annual amounts of precipi tation in period from a sample of 9 basins. These basins were denoted by 2, 26, 27, 40, 45, 48, 61 and 71 in Appendix 2 and Fig. la. First, the annual basin vaiues taken from the total data were estimated from isoiine anaiyses of maps of annual precipitation (The Finnish Meteorologicai Institute ). By multi plying these by 1+k and averaging over the years, corrected and unbiased amounts of mean annuai precipitation were obtained basinwise. By sub tracting from these the corresponding uncorrected values of the accepted material, unbiased totai corrections for the latter were obtained, the mean correction being 85.2 mm. The corresponding mean of biased correction was mm. By substracting the mean adhesion correction (Tzr 26 mm) from both corrections, the corresponding average windpiusevaporation corrections became 59.2 and 79.6 mm, their ratio (0.74) being the reducing factor for biased vaiues of b. The standard error of their difference (5 mm) divided by the latter correction approximates to the standard error of the reducing factor (0.06.). This error, corresponding to an inaccuracy of 3 to 4 mm in annuai precipitation, is negiigible. To ensure accuracy, the effects of ciimate on changes in b between the two periods were studied around the lijoki basin, where this effect was obviously greatest. Observations at Taivaikoski were used. In winter (November 1 to April 30), observed precipitation at this station was as much as 53 mm greater in period than in period In addition, only in northern Finland was the snow fali period (October 1 to May 31) colder in period than , this difference in the mean temperature being 0.54 C at Taivalkoski. For both reasons, the percentage of annuai precipitation falling as solid (denoted by p) was greater in period than in period The monthly mean temperature is denoted by t( C). To obtam monthly differences

11 typical typical 32 in p between the periods, monthly values of ap/ at were estimated by using both changes from month to month and geographical differences between the stations at Taivalkoski and Kajaani. Only in October and April was the derivative significant (5 to 6), while in midwinter it was negligible. As the mean precipitation values during period at Taivalkoski were corrected monthly by taking into account also the changes of p between the periods, we obtain the mean annual precipitation as being 797 mm. By using the standard correction for period , we obtain the resuit 793 mm. Thus, the climatological correction is 4 mm or smaller, generally about 2 mm. To observe this correction, biased values of b were multiplied by 0.76 in stead of Thus, corrections in mm based on the values of k in the map of Fig. 6 were ob tained by multiplying the observed amounts of precipitation by 0.76 b and by adding 0.24.T 6.2 mm to the resuit. The mean ± standard deviation of correction was 90±9 mm. The following estimates for the standard errors can be given (n = number of stations in the basin) typical large basins (F = km2, n = 12) 10 mm medium size basins (F km2, n= 5)14 mm small basins (F = km2, n 1,5) 27 mm Because the map analysis and basin values were corrected by the same method, no disagreement between them was observed (Fig. 3, Appendix 2). The map analysis of P was controlled in northern Lapland by the sums of runoff and evaporation (Equation 2. Section 6.1) for the basins 80, 81 and 82 (Fig. la, Appendix 2). The agreement was good. Fig. 5. Division into regions characterizing the classes of seasonal changes of precipitation from the period to the period as follows: A = decrease in early summer, increase in autumn B = decrease in early summer, increase in autumn and winter C = increase in autumn and winter D = no marked changes in any season 4.2 Precipitation during the periods and The map analysis of the change of annual precipi tation from period to period (Fig. 11) was made by using stationwise values. The sample of stations comprised the synoptical and climatological stations of per manent location and in addition some permanent precipitation stations. Regarding changes of precipitation in different seasons and per year, it was possible to identify regions with four different kinds of change (Table 3 and Fig. 5). In those basins in which the main components of water balance were calculated for both periods, precipitation increased on average 28±25 mm (mean ± standard deviation). Class C occurs in two separate regions of central and northern Finland (Fig. 5). In these regions P increased by about 50 mm. Of this increase 2/3 occurred in winter and 1/3 during the infiitration period of subsurfacewater storage, whereas during the depietion period of subsurface water storage the change was negligible. In class D, prevailing in Lapland northwest of region C, marked changes

12 5±16 33 did not occur in any season. Classes A and B prevail between and south of the regions of type C. In both elasses, P decreased in the depietion period of subsurface water storage by about 20 mm. In the infiitration period of subsurface water storage precipitation increased in both classes, but markedly (by about mm) only in class B. In winter precipi tation increased by about 20 mm in class B but did not change in class A. Consequently, the annual change was marked (about 30 mm) in class B but not in class A. Fig. 6. Total correction of precipitation (% ofobserved) for stations with an exposure of 35 %, which is the mean among a non selected sample of stations (btoken lines) and northern and southern boundaries of the region formed by the northern and middle zones of aapa boga. Solid lines with AAPA at their ends. 4.3 Temporal variation in the mean annual, January and July values of precipitation The temporal variation in the annual amounts of precipitation was examined for the same basins as for runoff (Section 3.4) by using un corrected values (Hydrografinen toimisto 1935, 1936, 1938, 1944, 1948, Tie ja vesirakennus hallitus 1954, 1957, 1962, 1963, 1965, 1968, 1970, National Board of Waters 1972, 1975, 1976, 1977). The standard deviation of the annual amounts of precipitation in periods and , denoted by S and their differences, denoted by d S are shown in Table 4. During period , S was about equal to or slightly smaller than in period in the basins Vuoksi, Imatra (Lake district, eastern part); Vantaa, Oulunkylä (Transition zone); Kyrönjoki, Lansorsund (Eastern Bothnian region) and Kalajoki, Hihnalankoski (Eastern Bothnian region). In the other basins (Kymijoki, (P0) (P0), (P0) (P0), Table 3. Changes of mean annual and seasonal amounts of precipitation from the period to the period (dp), classified according to the seasonal course as A, B, C and D. For each class, results are given as mean ± standard deviation of the station values. Class Number of Year V VJI viii xii) xii iv2) (region in Fig. 5) stations A 10 8±16 22± 6 12±10 2± 7 B 9 31± 9 18± 7 28± 6 22± 8 C 7 51±18 22±14 34±13 D 2 2± 5 8± 6 8± 2 2± 9 1) = Kuusamo (C), Taivalkoski (C), Sodankylä (D) VIII X 2) = Kuusamo (C), Taivalkoski (C), Sodankylä (D) Xi IV F

13 34 Table 4. Standard deviation of P0 denoted by S (P0) in the years of periods and and their difference denoted by d S (P0). Basin S (P0) mm d S (P0) mm Vuoksi, Imatra Kymijoki, Kalkkinen Vantaanjoki, Oulunkylä Kokemäenjoki, Harjavalta Kyrönjoki, Lansorsund Kalajoki, Hihnalankoski lijoki, Merikoski Kemijoki, Isohaara Average Table 5. Standard deviation of P0 denoted by S (P0) in January and in July durmg the periods and Basin 5 (P) in January 5 (P,) in July Vuoksi, Imatra Kymijoki, Kalkkinen Vantaanjoki,Oulunkylä Kokemäenjoki, Harjavalta Kyrönjoki, Lansorsund Kalajoki, Flihnalankoski lijoki, Merikoski Kemijoki, Isohaara Average Kalkkinen and Kokemäenjoki, Harjavalta in the western part of the Lake district and lijoki, Meri koski and Kemijoki, Isohaara in the Northern Bothnian region) S (P0) decreased from period to period by 15 to 29 mm. However, these changes were not statistically significant at the 5 % level. Additionally, the values of S (P0) were calcu lated for both periods in January and in July (Table 5). Injanuary the change in S(P 0) between the periods was marked only in the basins Kalajoki, Hihnalankoski and lijoki, Merikoski, both located between the latitudes and 66 ON. In the latter basin, the change was statistically significant at the 5 % level. In the areas around these basins, mean precipitation during winter increased markedly. Consequently, this increase was caused by the occurrence of winters of abundant precipitation during period but not during period ; on the other hand, the frequency of dry January months remained unchanged. Farther north (basin Kemijoki, Iso haara) S (P0) remained small. S (P0) was greatest in the basin Vantaanjoki, Oulunkylä; great variation of P0 in autumn and winter are typical only for the southern coast, due to orographical factors. In the four other basins of southern and central Finland, ail values were between 12.9 and 15.9 mm during both periods. Duringperiod , therefore, itwould appear that intensive cyclone centres occurred in January up to 630 of latitude and during period to about three latitude degrees further north. In July no marked differences in S (P0) were found between either the periods or the regions.

14 35 5. CHANGES IN WATER STORAGE The mean annual change of water storage between the beginning and the end of period was estimated by considering the water leveis in the lakes and the snow cover. For the changes in water storage in the lakes (ML), the average change in lake water level in the basin was approximated by that in a dominating lake or by averaging over several large lakes (detailed in Appendix 1). For the change of snow storage (MS), the values of water equivalent in large basins on January (Tie ja vesirakennus hallitus 1963) and on January (National Board of Waters 1980) were used (details in Ap pendix 1). The role of the change in water storage was mostly insignificant: the mean ± standard deviation of the basin values of MS ML (n = 72, Appendix 2), was 1 ± 5 mm. Consequently, the effect of temperature on evaporation could be neglected. 6.2 Influence of the amount of growing stock on evaporation Evaporation increases with the amount ofgrowing stock as indicated by Equation 2 in section 6.1. The contribution of this effect to the change of evaporation from period to period (d Ek, mm) could be obtained as the corresponding change in the term (1 L). Kd of the equation. Consequently, the mean amount of growing stock on land area (Kd, m3 hal) was needed on an average during both periods (Fig. 7. ). By denoting these values of Kd by Ka and Kb respectively, one obtains Ka = C2K 2+C3K3 (3) Kb =C5K5+C6K6 (4) 6. EVAPORATION 6.1 Influence of temperature on evapor ation during the period According to the evaporation equation of Solan tie (1975) the annual evaporation (mm) can be given as: Ek = G+1.14(1 L).Kd+L.EL (2) where G = the sum of effective temperature or the sum of daily mean temperature excess above +5 C L = the proportion of lakes in the basin area Kd = amount of growing stock on land area (m3 ha 1) EL = the term for lake evaporation (mm a1) The change in G from period to period was calculated by taking the sum of the differences between daily mean temperatures (temperature statistics: Kolkki 1966, Heino 1976). As a resuit, the values of this change and of the corresponding changes in Ek became negligible, the former being in ail basins within the limits ± 30 C d and the latter within limits ± 10 mm. These climatological changes, being so small, can hardly be separated from the effects caused by the changes in the observation sites. where K2, K3, K5 and K6 are values of Kd ac cording to the 2nd, 3rd, Sth and 6th national forest inventories (Ilvessalo 1957a, Kuusela 1967, Kuusela and Salovaara 1968, 1969, 1971, 1974, Kuusela and Salminen 1976, 1978) and C2, C3, C5 and C6 their weights. The basin values of K were approximated by the corresponding values by forestry board districts or by their weighted means (details in Appendix 1). The values of K3 were also given for combinations of basins and the values of K1 and K2 only as values for combinations of basins (Ilvessalo.90 o3 o = <0 m3ha 60 Southern Northern Whole of Finland 658 Inventory Vaara ±ig. 7. Course of the amount of growing stock in Fin land from 1923 to 1975 (m3 hai). Regarding changes, one m3 hal corresponds to an evaporation of about 1.14 mm). 5

15 1957b). When denoting the values of K for forestry board districts by K and those for combinations of basins by K, (Appendix 1), values of K 2 may be converted into values of K2 by the equation K2 = K2.(K3 /K 3 ) In order to obtain the weights in equations (3) and (4) from a graph showing K as a function of time, the values of K1, K2, K3, K4, K5 and K6 were calculated for Finland as a whole and for its southern and northern halves (the latter comprises the four northernmost forestry board districts). These values of K3, K4, K5 and K6 were obtained as means of the values of K3, K4, K5 and K6. In order to obtain corresponding values of K1, and K2,, corresponding values of K1, K2 and K3 were first calculated as means of the values of K1, K2 and K3. The final values of K1 and K2 were then obtained by multiplying K1 and K2 by the values of K3 / K3, which were for the southern half and for the northern half of Finland. Assuming that K1...K 6 represent values of K in the middle of the period of the first... sixth inventory, a curve of K as a function of time could be obtained for Finland as a whole and for its halves. By graphical integration, the corresponding averages for the periods and could be obtained. In equations (3) and (4) the coefficients became equal in both halves of Finland, being: 0.50, C3 = 0.50, C5 = 0.75 and C6 = By applying equations (3) and (4) to individual basins, the change in the basin values of Kd from period to period could be obtained, as well as the basin values of dek (Appendix 2). In those 39 basins in which the water balance could be obtained for both periods, the mean ± standard deviation of dek was + 3 ± 6 mm. 6.3 Influence of dry summers and of winters with abundant snow cover on evaporation during the period Equation (2) in Section 6.1, approximating potential evaporation, does not take into account the effect of soil water deficit on evaporation. In 36 Finland, this effect is obtained only in the hemiboreal and southern boreal vegetational zones (Fig. 9) or hydrologically in the Baltic regime, the transition regime and the regime of the Lake district, where the climate of the growing season is the driest. The effect is notice (5) able in dry summers, particularly when preceeded by winters of small snow storage. Such winters usually occur only in regions where the meän annual maximum of the water equivalent of the snow cover is less than 100 mm (Solantie 1981). Particularly in the regions where the change in precipitation between periods and was of class A (Fig. 5), such areas of thin snow cover reached farther inland from the coast and farther northward along the coast in period than in period , in Ostrobothnia reaching even into the region of the Eastern Bothnian regime (Figs. 8 and 9). Of the 67 basins for which the value of Eb Ek could be obtained, more than half of ail five basins, denoted by 31, 32, 33,43 and 44 in Fig. 1 and in the Appendix 1, is situated in those parts of the hemiboreal and southern boreal zones in which the mean maximum snow storage is less than 100 mm. The major part of basins 45 and 46 is located just north of the common northern boundary of the southern boreal vegetational zone and the transition regime. However, in period , during which summers in these basins were especially dry, the climate there was temporarily similar to that characteristic of the southern vegetational zone. Further, because in the major parts of basins 45 and 46 the mean maximum snow storage was less than 100 mm, they were also classified as dry basins, thus making a total of seven. Actual evaporation can be approximated by EB and potential evaporation by Ek. Evaporation is greatest in forests, increasing with the amount of growing stock (Equation 2 in Section 6.1). Therefore, the deviation in actual evaporation from the potential value was obtained for each basin by regression analysis, using the mean amounts of growing stock in the forests denoted by x as the independent variable. The dependent variable was also given for forest areas, not for total areas. Consequently, parameter y = (EB Ek):((1 L). md) was used instead of EB Ek. Here, (1 L). maj, indicating the proportion of forest of the total area (denoted by m), is given as a product of the proportion of land of the total area and the proportion of forests of the land area (md). Negative values of y indicate

16 estimated 37 Fig. 8. Northern boundary of the southern boreal vegetational zone (broken line) south of which early summers are generally drier than north of it (see also Fig. 9). = region in which summers, particularly in the period , were more humid than generally = region in which scant snow storage was connected with dry summers in the period ; north of the broken line summers, particular]y in the period , were less humid than generally. Fig. 9. Hydrological zones and regions together with the vegetational zones. Hydrological zones and their division into hydrological regions (thin continuous lines): A = Baltic zone B = transitiofl zone C = inland zone = Lake district C2 = Eastern Bothnian region = Maanselkä region C4 = Northern Bothnian region D = fjeld zone D1 = Lapland region = Kilpisjärvi region Vegetation zones (broken lines): HB = hemiboreal zone SB = southern boreal zone MB = middle boreal zone NB = northern boreal zone that actual evaporation is smaller than the potential value. The values of x and m were by map analyses based on results of the 5th inventory (Atias of Finland 1976). These values are shown basinwise in Table 6. As a resuit, a regression equation was obtained: y = x (6) Coefficient of correlation = O.714(significant at a 5 % level). Standard error of estimate of y on x = 29 mm. Percentage of explained variance = 51 % F

17 38 The pair (y = 0, x = 86) indicates that as the mean volume of growing stock in the forest exceeds around 86 m3 ha oration from the forests becomes smaller than the potential value. By multiplying this value by the mean value of m, it was found that this limit corresponds to 50 m3 ha area. In the three of these 7 basin, having the greatest value of x (basins 33, 43 and 44 in Table 6, ali belonging to the main basin Ko kemäenjoki), the theoretical deviation of the actual evaporation from the potential during period was in forests 51 mm and in the whole basin area 30 to 34 mm. The values of EB Ek, being 19 to 41 mm, agree weli with the iatter figures. For the 5 basins in which x > 86 m3 ha EB Ek from 0 was significant at the 10 % level. Consequently, even during periods of unusually dry summers in the hydrologically driest basins, the infiuence of drought on evaporation is hardiy noticeable because of the rather humid climate of Finland. In addition, the soil in forests of the densest stand is mostiy moraine inciuding ciay or even pure clay and therefore has a good field capacity. After winters rich in snow, aapa bog com piexes do not always rid themseives of snow meit waters in summer. Particularly in the aapa bog compiex zones of Eastern Bothnia and Southern Lapland (Ruuhijärvi and Hosiais luoma 1981, Fig. 6), in which aapa bogs are the widest and wettest (Ruuhijärvi 1960), wide water surfaces occur. Because on aapa bog complexes the bottom layers of loose Carex peat are somewhat isoiated from meit and 1 the actual evap 1 of the total 1, the mean deviation of rainwater by a rather dense peat layer having a poor vertical water conductivity, the depth and width of the water layer on a bog varies mainly according to the infiow of meit waters from the surroundings of the bog. In the aapa bog compiex zones of Eastern Bothnia and southern Lapiand the water surfaces were in period obviously either wider than (in regions of ciass C, Fig. 5) or approximately as wide as (regions of class B and D, Fig. 5) those prevailing in period The influence of these water leveis on evaporation in period was studied by regression analysis, in which basin values of EB Ek in period were obtained as functions of the proportions of open bogs (denoted by p) in the basin areas. Ali the basins in these two zones of aapa bog complexes in which p is at least 5 % were inciuded. In this way, only the basins near the southern boundary of the aapa bog compiex zone of Eastern Bothnia, belonging to the southern boreal vegetationai zone, were negiected. In addition, the basins 50 and 56, where p is rather small, were inciuded without dividing them into parts. The vaiues of p were obtained by muitipiying the proportion of land of the totai basin area by the proportion of peatiands of the iand area and by the proportion of open bogs of the peatland area. The two latter factors were estimated from a map anaiysis by Iivessaio (1960) based on results from the 3rd forest inventory. In these 23 basins rich in open bogs EB Ek ± S(EB Ek) = 25.9 ± 21.7 mm and pn ±S(p) = 15.2 ± 7.5 % Table 6. Values of y indicating deviation of actual evaporation from the potential evaporation in forests of the 7 dry basins, where this deviation is expected to be negative durin the period , and corresponding values of x indicating the mean volume of growing stock in the forests (m3 ha difference between the estimates of actual and potential evaporation (EB Ek) by the proportion of forest of the total area (m). The proportion of forest of the land area (md) is also given. t). Values of y were obtained by dividing the Basin md m y=(e E)m x (numbers refer to Fig. la % % (mm) (m3 hal) and Appendix 2) Mean Standard deviation

18 39 As a resuit from the regression analysis we obtain EB Ek (mm) = p (7) Coefficient of correlation = (significant at a 1 % level). Standard error of estimate of EB Ek on p = 17.7 mm. Pereentage of explained variance 31 %. The basins included in this regression analysis are set apart in Appendix 2 by having values entered for p, for the increase of evaporation due to this effect (EknEk, where Ekn Ek pn) and for the error of estimate (EB Ekn). In these basins, on average, Ekn Ek was equal to EB Ek = 26 mm. By setting p = 100 in equation (7), one finds that in period a partial water cover on aapa bogs increased the annual evaporation there by about 163 mm. According to equation (2) in Section 6.1, annual evaporation on treeless areas (kd 0) is in these zones of aapa bog complexes about 250 mm. About 3/4 of this evaporation (about 190 mm) occurs during the period between the date of final thaw of snow cover over open bogs (about May 10) and the end of September. One may approximate evaporation from the water surfaces of such bogs by the mean evaporation from Class A pans of the National Board of Waters, Finland between the 640 and 68.5 N lines of latitude. Thus approximated, mean evaporation during the period June 1 to September 30 in the years amounted to 357 mm (National Board of Waters 1980). Parts of the Class A pan obser vations were aiready commenced in May. Using these, the mean evaporation for the period May 10 to 31 can he estimated as being about 40 mm. The total evaporation from pans during the period May 10 to September 30 was about 390 to 400 mm. Consequently, on such bogs, the annual evaporation from watercovered surfaces is about 210 mm greater than from bog surfaces not watercovered. This value is not much greater than the estimate of the corresponding addition to the evaporation on open bogs during period caused by the water partially covering them (163 mm as given above). Considering that the increase in evaporation spread over the total area of the region in question (the two zones of aapa bog complexes) is 26 mm, and that the evaporation in the basins of this region was in period on average 19 mm less than in period (valuesofdeb in Appendix 2), the increase due to water cover for the total area of the region was in period only about 7 mm. Consequently, in summer, water obviously covered on average 3/4 of these bogs in period but only 1/4 inperiod In agreement with this, the water equivalent of the snow cover on the mean date of the winter maxi mum, April 1, denoted by V, was during period on average 22.7 mm smaller in this region than in period Consequently, we can obtain the change of EB Ek with Vas being a (EB Ek)/a V = p, where regional means of basin values are denoted by bars. It is now possible to obtain a common solution of an equation giving EB Ek as a linear function of V. Equation (7) represents a single solution of this function, being valid when V is equal to the mean in this period, denoted by Vb. Consequently, EB Ek ( p) = (V Vb). from which EB Ek = Pn (V + 30 Vb) (8) Now solve the equation for V in the cases a) Pn = 100%, EB Ek = 1 mm (no water on peat) b) pn 100%, EB Ek= 2lOmm (peat totally watercovered). In case a), V = Vb 30 mm. For even smaller values of V, EB Ek = 0. In case b), V = Vb + 8 mm. On the other hand, for even greater values of V, EB Ek = 210 mm. In this equation, Vb is intentionally made to vary basinwise. In order to use this equation, the values of Vb can be read off the map analysis for the mean annual maximum of the water equivalent of snow cover during the period (Solantie 1981). The arguments for this are as follows: The development and preservation of aapa bog formation are caused firstly by water supply from surrounding lands (Ruuhijärvi 1960) and secondly by conditions being so humid in early summer that water remains on the peat throughout the season. Further, the minimum width of the water surfaces in summer should vary annually from 0 to 100 %, shallow water layers being typical. Similar meit water inflows on bogs per unit area of bogs can occur in regions of different mean values of V. The lower limit of the relative area of the mineral lands surroundings the bog in creases as the mean value of V decreases. There fore, aapa bogs become ever more common

19 40 going northward. The southern boundary of aapa bog complexes is however mainly determined by the difference between mean evaporation and precipitation in June (Solantie 1974). Firstly, in the southern boreal vegetational zone, evap oration in the forests with greater amounts of growing stock than in more northerly zones is from the snow meit onwards intensive enough to decrease rapidly the runoff into the bogs (Solantie 1974). Secondly, the difference be tween precipitation and evaporation on the bogs is in June perhaps a more important reason for the occurrence of aapa bogs. In eastern Finland, where June is somewhat more humid, aapa bogs therefore also occur in the northernmost part of the southern boreal vegetational zone, but farther west they are absent in the southern and western parts of the middle boreal vegetational zone, where the vicinity of the sea reduces precipitation in June (Solantie 1974). Because weather conditions during summer are also important for the width of water surfaces on bogs, equation (8) is applicabie over periods of several years. Further, storage of water for several years may occur in aapa bog compiexes. 6.4 Changes in mean annual evaporation between the periods and For the basins of the aapa bog zones of Eastern Bothnia and southern Lapland, in which the main components of the water baiance could be caiculated both for period and period (n=11), the foliowing changes from to were obtained as values of mean ± standard deviation (mm): dr deb dp +27±24 +19±20 +47±27 The corresponding values for basins outside these regions (n28) were dr deb +23±25 4±26 and those for ali the basins (n=39) dr deb +24±25 +2±27 dp +20 ± 19 dp 128 ± 25 in the region of aapa bog complexes annual precipitation increased about30 mm more than elsewhere. Taking into consideration the fact that the mean value of dek, indicating the in fluence of changes of wood stand on evaporation, is in this region 2 mm, deb is practically totally caused by the variation in bog evaporation. Consequentiy, aapa bog complexes somewhat re duce the iongterm variation in runoff. In areas outside the aapa bog regions the effect of changes of wood stand was aiso in significant (mean ± standard deviation of dek was + 3 ± 6 mm in the basins for which deb was obtained). Ali other effects are also generaily negligibie, because the mean of deb dek did not deviate from 0 even at the 90 % level of significance (mean ± standard deviation 8 ± 26 mm). If the effect of dry summers is further eiiminated using equation (6), the mean change is even more insignificant (mean ± standard deviation being 6 ± 26 mm). For ali basins in Finland for which deb couid be obtained, the mean ± standard deviation of this parameter was + 2 ± 27 mm. Thus evap oration did not change noticeably from period to period Consequently, the increase of precipitation is oniy refiected in an approximately equal increase in runoff. In the light of the theories presented in this study, annual evaporation varies less than annual precipitation. As the standard deviation of the sum of effective temperatures (G) is in iong period series about 130 C, the corresponding standard deviation of Ek (Equation 2, Section 6) is for 30 year means 7 mm and for the difference between two such means 10 mm. The standard deviation of annual corrected precipitation is about 100 mm. For example, in the sample of 9 basins in Section 4.1 the standard deviation of the mean of annual precipitation was 97 ± 4 mm (average standard deviation ± standard deviation among basins). Consequently, the standard devi ation of the 30 years mean of precipitation is about 18 mm and that of the difference between two such means about 25 mm. Runoff in Finland is thus approximateiy the difference between two mutually independent variabies. The standard deviation of its 30 years mean is about 21 mm and the difference between two such means about 30 mm. The estimates of the magnitude of the variation of R and EB presented in this section agree well with the results for a sample of basins in Sections 3.4 and Confidence level of evaporation By comparing evaporation obtained by the water balance equation method (EB) with that obtained

20 41 with the evaporation equation of Solantie (Ek, Equation (2)), the mean ± standard deviation of their difference is in the aapa bog region (23 basins, basins 50 and 56 undivided, Fig. la and Appendix 2) 26 ± 22 mm, in other regions (44 basins) 6 ± 20 mm and in the whole of Finland +5 ± 25 mm; this both in the case that basins 50 and 56 are divided into parts (n=72) or treated as wholes (n=67). If the effect of water surfaces of bogs on evaporation is further taken into account (Equation 7), the error of estimate (mean ± standard deviation) is in the aapa bog region (n=23) 0 ± 18 mm and in the whole of Finland (n=67) 4 ± 19 mm. By further taking into account the effect of dry summers (Equation 6), the corresponding values are outside the aapa bog region (n=44) 3 ± 18 mm and in the whole of Finland (n= 67) 2 ± 18 mm. Consequently, the evaporation equation by Solantie (Equation 2), particularly when applied with correction for the effects of water on bogs and for dry summers, renders a mean evaporation in the period without any systematical errors. 6.6 Laying out of the map of mean annual evaporation during the period The draft evaporation map was fitted in detail on the basis of the values of Ekn, values of Ek (Equation 2) being corrected by the effect of water on bogs according to Equation 7. However, a basinwise check was carried out to ascertain that the analysis was in agreement with the values of EB, and isolines were shifted a little where necessary. 6.7 Temporal variation in the annual amount of evaporation Considering that the standard deviation of the sum of effective temperatures is about 130 C d, the standard deviation (S) of the annual amount of evaporation (E0) according to Solantie s evap oration equation (6.2) is about 40 mm. The value of S (E0) was also calculated using annual values of EB for the period for a sample of basins (Table 7). To obtain E0 for this purpose, uncorrected basinwise values of P0 were used. In order to avoidthe problernatics of water storage in lakes, only basins of a small relative lake area were included out of those for which S (R0) had been calculated. Consequently, the three basins of the Lake district (Section 3.4) were discarded. The annual ehange in water storage (M0) was approximated by that in snow cover on the basis of the values of the water equivalent of the snow cover on January 1 (Hydrografinen toimisto 1935, 1936, 1938, 1944, 1948, Tiejavesiraken nushallitus, 1954, 1957, 1962, 1963, 1965, 1968, 1970, National Board of Waters 1972, 1975, 1976, 1977). To obtain the variation of M0, S (M0) was also calculated. Because snow stores were less well known during period , values of E0 could not be determined for this period. The values of S (E0) agree well with the rough estimate based on Eq. (2). The generally small dependence of evaporation on precipitation in the humid climate of Finland (Solantie 1975) is reflected in these results. However, the dryness of summers in southern Finland and the abundance of water surfaces on bogs in the middle zone of aapa bogs, both of which accentuated particularly in the period , showupascorrelations somewhat higher than 0. The effect of dryness shows well only inside the dry region (Vantaan joki, Oulunkylä) but not at its northern boundary (Kyrönjoki, Lansorsund). This is also seen in the variance of the other water balance components. If it is assumed that R is the sum of the indepen dent variables P0, E 0 and M 0 so that S (E0) = J s2 (P0) S 2 (M0) + S2 (R0), the resuit for the basin Vantaajoki, Oulunkylä would he imaginary, whereas the corresponding resuit for the Kyrön joki, Lansorsund (56 mm) is of the magnitude of the observed value (37 mm). By approximating values of S (M0) during period by those for period , we also obtain the values of S (E0) according to such an assumption for the period The result, 10 mm for the basin Vantaajoki, Oulunkylä, agrees well with the fact that during period summers were more humid than during period The result for the basin Kyrönjoki, Lansorsund (49 mm) is also realistic. Of the three northern basins, Kalajoki, Hihna lankoski is located around the southern boundary of the aapa bog region but lijoki, Merikoski and Kemijoki, Isohaara are well within it. In the two latter basins, S (P0) > S (R0) in both periods and ; consequently, Corr. (P0, E0) is in this area significantly positive also in period In the basin Kalajoki, Hihnalankoski S (P0) > S (R0) and Corr (P0, E0) > 0 in the period as in the other two basins, because abundant waters covered the

21 42 Table 7. The values of the standard deviation of the annual amount of evaporation (E0), denoted by S (E0), the correlation coefficient between the annual amount of precipitation (P0) and (E0), denoted by Corr. (P0, E0), the percentage of the variance of E0 depending on P0, denoted by F (P0) and the standard deviation of the annual change in water storage, (denoted by S (M0). Basin Hydrological zone/ S (En) Corr.(P 0, E0) F (P0) S (M0) region (mm) (mm) Vantaanjoki, Oulunkylä Transition zone Kyrönjoki, Lansorsund Eastern Bothnian region Kalajoki, Hihnalankoski Eastern Bothnian region lijoki, Merikoski Northern Bothnian region Kemijoki, Isohaara Northern Bothnian region Average bogs, but only to a lesser degree. In period , during which the bogs were less covered by water, the value of S (E0) for the basin Kala joki, Hihnaiankoski, obtained by assuming that R is a sum of three independent variabies, is 43 mm. This appears to be a realistic magnitude. Consequently, both the regional and temporal differences in Corr (P0, E0) agree weli with the effect of aapa bog complexes on evaporation. As a conclusion, in two regions of Finland in the southern and in the northern parts of the country, Corr (P0, E0) is slightiy positive, the degree of dependence being sensitive to ciimatological changes. The southern region comprises the Baitic zone, the transition zone and obviously also the southern part of the Lake district, whereas the northern region comprises the southern and middle zone of aapa bogs. Between these regions, there is a zone in which evaporation is in practice totally independent of precipitation. In cool and wet summers, short warm periods and low temperatures reduce evaporation to about the same extent as inter ception on branches increases it, taking into consideration the rather smail amount of growing stock in this area. Even in warm and dry summers peat, occurring commonly in this region, retains moisture sufficientiy to maintain a potential evaporation. 7. SUMMARY In thisstudy; annual means of runoff(r), precipi tation (P) and evaporation (E) were obtained for the years both basinwise and as isoline analyses on maps. Precipitation was cor rected for measuring errors but not stationwise. Evaporation was calculated by two methods. Firstly, it was obtained as a residue from the water balance equation taking into account the changes of water storage in lakes and snow cover. Secondly, evaporation was caiculated by the evaporation equation of Solantie (Ek), approxi mating E as a function of the sum of effective temperature and the volume of growing stock. This equation was improved by adding terms taking into account the corrections for bogs being partially flooded and on the other hand for heavily wooded land where drought may reduce evaporation. After correcting the values of Ek by these two corrections, ali systematicai errors were practically eiiminated. For ali basins in Finland, the mean ± standard deviation of the vaiues of EB Ek was without the additional corrections 5 ± 25 mm and with them 2 ± 18 mm. Consequentiy, the overaii resuits improved somewhat with the correction. The standard error of EB and Ek should be smaiier than the standard deviation of EB Ek. The water baiance in Finland in the period was obtained as mutually independent components for 72 separate basins, covering aitogether 74 % of the area of the country. Some small basins between discharge measurement sites up and downstream were first rejected on the basis of insufficient accuracy of P and E. The change in values of the components from the period to the period couid be obtained for 39 basins. The mean ± standard deviation of this change was for P +28 ± 25 rnm,forr +24±25 mm and for E +2 ±27 mm. Precipitation and runoff increased about equaliy (Fig. 10 and 11), but evaporation did not

22 43 Fig. 10. Change of runoff (mm a1) from the period to the period Fig. 11. Change of corrected precipitation (mm a1) from the period to the period change much. In agreement with the change in EB, Ek remained unchanged because the amount of growing stock and the mean annual sum of effective temperatures were also unchanged. remarks and prof. Seppo Mustonen for back ground advice of the work. Helsinki, October 1984 Reijo Solantie, Matti Ekholm ACKNOWLEDGEMENTS The contribution concerning runoff was made by Matti Ekholm, that concerning precipitation and evaporation by Reijo Solantie and the remainder of the work was carried out in cooperation. The authors thank Mr. Svante Nordström and Michael Bailey for linquistic revision, Mr. Veli Hyvärinen and Ms. Raija Leppäjärvi for useful LOPPUTIIVISTELMA Tässä tutkimuksessa laskettiin vesitaseen pää komponentit sadanta (P), haihdunta (E) ja valun ta (R) keskimäärin kauden vuotta kohti sekä valumaalueittain (liite 2) että kartta analyyseinä (kuvat 2 4).Tuloksiaverrattiin myös kauden vastaaviin arvoihin. Sadanta korjattiin tarttumisvirheen sekä tuuli ja haihtu

23 44 misvirheen osalta. Jo tämän kauden keskimääräi sen korjaamattoman vuosisadannan karttoja laa dittaessa ja aluearvoja laskettaessa (Vesihallitus 1980) oli hylätty sijainniltaan avoimimmat ase mat (20 % kaikista). Sovellettaessa tähän aineis toon kaikkien asemien aineistolle laskettuja kor jauskertoimia (Solantie 1976), tulevat arvot yli korjatuiksi. Ylikorjauksen selvittämiseksi lasket tiin 9 valumaalueen otokselle korjattu vuosisa danta keskimäärin kautena myös valikoimattomasta aineistosta (perusaineistona vuosisadann an karttaanalyysit sarjassa Sade ja lumihavainnot ). Näiden korjattujen aluearvoj en ja vastaavien hyväksytyn aineiston korjaamattomien aluearvojen erotuksena saatiin todelliset korjaukset. Otosalueilla keskimäärin oli oikea korjaus 74 % ylikorjauksesta; ottaen huo mioon, että korjauskertoimet oli laskettu kau delle , kerrottiin ylikorjaukset muut tuneet ilmastoolot huomioiden 0.74:n sijasta 0.76:lla. Sadantoja korjattaessa katsottiin sade asemaverkko niin tiheäksi, että korjaukseen vai kuttavaa aseman avoimmuutta ei tarvinnut huo mioida asemakohtaisesti. Valunta laskettiin 72 erillisellevalumaalueelle, jotka käsittivät yhdessä 74% Suomen pintaalasta. Alueiden sisälle ja välille saatiin yksityiskohdat sadanta ja haihduntakartan avulla. Valunnat las kettiin vain alueille, joilla myös sadanta saatiin erittäin tarkoin (paitsi Enontekiöllä ja Utsjoella, missä sadanta laskettiin eräille alueille valunnan ja arvioidun haihdunnan summana). Vesitase jätettiin laskematta pienimmille ns. välialueille eli alueille, joita myös virtaamamittauspaikat ylä juoksulla rajaavat. Toisaalta eräitä sadanta ja haihduntaarvojen kanssa ristiriitaisia valunta arvoja tutkittaessa ne havaittiin epätarkoiksi ja tarkennuksen jälkeen vesitaseeseen hyvin sopi viksi. Myös vesivarastojen muutokset kauden vuotta kohti arvioitiin lumi ja järvivarastojen muutok sina. Ne osoittautuivat merkittäviksi vain har voissa tapauksissa. Haihdunta laskettiin sekä jäännösterminä vesi taseyhtälöstä (EB) että siitä riippumattomana parametrina Ek ns. Solantien (1975) haihdunta yhtälöstä: Ek= G+ i,14.(1 L). Kd+ L. EL, (1) missä G tehoisan. låmpötilan summa(kasvukauden vuorokausikeskilämpötilojen + 5 C:n yht tävien osien summa) L = järvien osuus pintaalasta (%) Kd = puuston määrä (m3 ha EL = järvihaihdunta (mm a1) 1) maaalueella ja Tälle yhtälölle laskettiin kaksi korjaustermiä selittämällä EB Ek:n arvoja regressioanalyysillä. Lisähaihdunta selitettiin Pohjanmaan ja Perä Pohjolan aapasuovyöhykkeissä (Ruuhijärvi ja Ho siaisluoma 1981; kuva 6) haihduntana turpeen päälle runsaslumisten talvien jälkeisinä kesinä jäävästä vedestä, riippumattomana muuttujana nevojen prosenttiosuus pintaalasta (pn): EB Ek 1 + 1,62 p (mm) (2) Valumaalueita oli 23 kpl ja EB Ek:n keskiarvo niillä 26 mm. Korrelaatiokerroin (0.56 1) on merkitsevä 1 % :n riskitasolla. Koska kausi oli näillä alueilla eri tyisen runsasluminen, oli aapasoilla vesipintaa ilmeisesti huomattavasti enemmän kuin esim. vähälumisena kautena Merkittäessä lumipeitteen vesiarvoa 1.4. eli maksimin keski määräisenä ajankohtana V:llä ja erityisesti sen keskiarvoa kautena Vb:llä, saadaan kausien ja V:n ja EB Ek:n arvojen kautta kulkeva suora, jonka yhtälö Ekn= Ek+1+0,055pn(VVb+30)(mm) (3) antaa yleisen ratkaisun Ekn nevahaihdunnan osalta korjatulle Ek:lle tarkasteltavan kauden tai vuoden V:n arvon ja kahden valumaaluekohtai sen vakion (Vb ja pn) funktiona. Vb:n on syytä olla valumaaluekohtainen siksi, että aapasoita syntyy niihin kohtiin, missä suolle sen valuma alueelta keväällä tuleva vesimäärä on suon pintaalayksikköä kohti tiettyä kynnysarvoa suurempi. Vähälumisilla alueilla vähimmäisvesi määrän edellyttämä suon ja sen valumaalueen pintaalojen suhteen yläraja on pienempi kuin runsaslumisilla seuduilla, mikä heijastuu lähinnä pn:n pienenemisenä kohti vähälumisia seutuja. Yhtälön (3) mukaan nevalla ei ole päällys vettä, kun V = Vb 30 mm tai pienempi, ja neva on veden peittämä, kun V = Vb+ 8 mm. Toinen korjaus Ek:n arvoihin tarvitaan kuivil la alueilla. Kuiva kesäilmasto vastaa Suomessa hemiboreaalista ja eteläboreaalista kasvilhisuus vyöhykettä eli hydrologisesti Itämeren vyöhy kettä, siirtymävyöhykettä ja JärviSuomen aluet ta (kuva 9); erityisen kuivakesäisenä kautena kuivan kesäilmaston alue ulottui Pohjanmaalla vähän pohjoisemmaksikin (kuva8). Hyvin vähälumisia talvia, joiden jälkeen kevät kosteuskin on vähäistä, sattuu alueella, jossa lumipeitteen vesiarvon keskimääräinen vuosimak

24 45 simi on alle 100 mm (Solantie 1981). Niillä 7 valumaalueella, joissa kautena kesän kuivuus ja talven vähälumisuus yhdistyivät, saa tiin kuivuuden vaikutusta kuvaamaan regressio yhtälö (EB Ek):m = Km missä m = metsän osuus valumaalueen pintaalastaja Km = puumäärämetsässä (m3 ha 1) Korrelaatiokerroin = (merkitsevä 5 % :fl riskitasolla). Yhtälön (4) mukaan kuivuus pienen tää haihduntaa, kun Km on vähintään 86 m3 ha. Suurinta Km:n arvoa 110 m3 ha vastaten (EB Ek):m = 51 mm ja EB Ek 3 2 mm. Kun Ek:n arvoihin tehtiin yhtälöiden (2) ja (4) mukai set korjaukset, muuttui EB Ek:n keskiarvo ± kes kihajonta koko Suomen aineistossa 5 ± 25:stä 2 ± 18:aan mm:iin. Sekä EB:fl että Ek:n keskivirhe on tietysti pienempi kuin EB Ek:n keski hajonta. Kaikenkaikkiaan vesitase selvitettiin 72 erilli selle alueelle, jotka käsittävät 74 % Suomen pintaalasta. Lisäksi vesitaseen pääkomponent tien muutos kaudesta kauteen voitiin selvittää 39 valumaalueella. Muutos (keskiarvo ± keskihajonta) oli sadannal le 28 ± 25 mm, valunnalle +24 ± 25 mm ja haih dunnalle (EB) + 2 ± 27 mm. Sadanta ja valunta kasvoivat suunnilleen saman verran (kuvat 10 ja 11), ja haihdunta pysyi suunnilleen ennallaan; vain aapasuoalueilla tuntui sadannan lisäys (47 ± 27 mm) myös haihdunnan lisäyksenä (19 ± 20 mm). Sopusoinnussa EB:n muuttumattomuuden kanssa ei Ek koko Suomessa keskimäärin muut tunut lainkaan (puuston määrä ja tehoisan läm pötilan summa jokseenkin muuttumattomat). 1 LIST OF SYMBOLS 1. Correction of precipitation T B = mean adhesion correction or error (mma1) = windplusevaporation correction or error (mma1) b = relative windplusevaporation correction (ratio of B to observed precipitation) k relative total correction (Tatio of B+T to observed precipitation) p = proportion of solid precipitation of the total t = monthly mean temperature ( C) S = For S, see symbols of group 4 below C = index, indicating that stations most exposed are neglected ( accepted material ). It is used only for avoiding confusion with total material = exposure of the observation site c = the component of o in a certain compass direction 2. Water balance components P R EB Ek = mean annual precipitation (mma1) = mean annual runoff (mma1) = mean annual evaporation from the water balance equation (mm a1) = mean annual evaporation from the Solan tie evaporation equation (mma1) MS = mean annual change of water storage in the lakes from the period to the period (mma1) ML = mean annual change of water storage in the snow cover froni the period to the period (mma1) M = mean annual total change of water storage from the period to the period (mma1) d = (in front of symbols) the change in the parameter from period to period Symbols connected with the determination of evaporation G = sum of effective temperatures being the sum of daily mean temperature excesses above 5 C during the growing season Km = mean amount of growing stock in the forests (m3. hai) Kd = mean amount of growing stock on the land areas (m3 ha for which the following syrnbols were also used in particular cases: 1),

25 46 Ka = mean of kd during the period Kb = mean of kd during the period K = values of kd for forestry board districts K = values of kd for combinations of basins Indexes 1,2...6 for kd and its particular symbols are used for the ordinals of the national forest inventories m = proportion of forest of the total area md = proportion of forest of the land area = proportion of open bogs of the total area V = water equivalent of the snow cover on April 1 Vb = value of V in the period EL = annual evaporation from lakes 4. General symbols S = standard deviation or standard error of any parameter, following S in parentheses L = proportion of lakes of the total area A,B,C,D= classes of change of precipitation (dp) F v = index indicating an intermediate basin, that is a basin bounded by flow observation sites both downstream and upstream VF = proportion of the area of an intermediate basin of the total basin area upstream of the downstream discharge site p = density of observations (number/unit area) = latitude n = number of cases = a symbol of dependent variabies in y gression analysis x = a symbol of independent variabies in gression analysis =area REFERENCES Ahti,, HämetAhti, L. & Jalas, J Vegetation and their sections in northwestern Europe. Ann. Bot. Fennici 5: Finnish Meteorological lnstitute Precipi tation and snow cover data (Sade ja lumihavainnot ). Meteorological year book of Finland 61:2 75:2. Heino, R Climatological tabies in Finland Supplement to the Meteorological Yearbook of Finland. (Taulukoita ja karttoja Suo men ilmastooloista kaudelta ). 75 :la. 41 p. Publications of the Finnish Meteorological Institute. re re Hydrografinen toimisto 1935, 1936, 1938, 1944, Hydrological Yearbooks Helsinki. Ilvessalo, Y. 1957a. Suomen metsät metsänhoitolauta kuntien toimintaalueittain. Valtakunnan metsien inventoinnin tuloksia. (English summary: The forests of Finland by forestry board districts). Comm. Inst. For. Fenn Ilvessalo, Y. 1957b. Suomen metsät päävesistöalueittain. Valtakunnan metsien inventoinnin tuloksia. (English summary: The forests of Finland by the main water systems). Comm. Inst. For. Fenn Ilvessalo, Y Suomen metsät kartakkeiden valossa. (English summary: The forests of Finland in the light of maps). Comm. Inst. For. Fenn Kolkki, Tables and maps of temperature in Finland during Supplement to the Meteorological yearbook of Finland. (Taulukoita ja karttoja Suomen lämpötiloista kaudelta ). 65:la. Publications of the Finnish Meteorological lnstitute. Korhonen, V Ein neues Verfahren bei der Korrek don des Schneemessungen. Ann. Acad. Scient. Fenn. Series A. 1. MathematicaPhysica 24. Kuusela, K Helsingin, LounaisSuomen, Satakun nan, UudenmaanHämeen, PohjoisHämeen ja Itä Hämeen metsävarat vuosina (English summary: Forest resources in the forestry board districts of Helsinki, LounaisSuomi, Satakunta, UusimaaHänie, PohjoisHäme and ItäHäme in ). Folia forestalia 27(2). 56 p. Kuusela, K. & Salminen, PohjoisKarjalan met sävarat vuosina , EteläPohjanmaan, Vaa san ja KeskiSuomen vuosina 1974 sekä Kainuun ja PohjoisPohjanmaan vuonna (English summary: Forest resources in the forestry board districts of PohjoisKarjala in , EteläPohjanmaa, Vaasa and KeskiSuomi in 1974 and of Kainuu and PohjoisPohjanmaa jo 1975). Folia forestalia p. Kuusela, K. & Salminen KoillisSuomen metsävarat vuonna 1976 ja Lapin metsävarat vuosina 1970 ja (English summary: Forest resources in the forestry board districts of KoillisSuomi in 1976 and of Lappi in 1970 and ). Folia forestalia p. Kuusela, K. & Salovaara, A EteläSavon, Etelä Karjalan, ItäSavon, PohjoisKarjalan, PohjoisSavon ja KeskiSuomen metsävarat vuosina (English suamary: Forest resources in the forestry board districts of EteläSavo, EteläKarjala, ItäSavo, PohjoisKarjala, PohjoisSavo and KeskiSuomi jo ). Folia forestalia p. Kuusela, K. & Salovaara, A EteläPohjanmaan, Vaasan ja KeskiPohjanmaan metsävarat vuonna (English summary: Forest resources in the forestry board districts of EteläPohjanmaa and KeskiPohjanmaa in 1968). folia forestalia p. Kuusela, K. & Salovaara, A Kainuun, Pohjois Pohjanmaan, KoillisSuomen ja Lapin metsävarat vuosina (English summary: Forest resources in the forestry board districts of Kainuu, PohjoisPohjanmaa, KoillisSuomi and Lappi in ). Folia forestalia lio. 49 p. Kuusela, K. & Salovaara, A Ahvenanmaan maa kunnan, Helsingin, LounaisSuomen, Satakunnan, UudenmaanHämeen, PirkkaHämeen, ItäHämeen, EteläHämeen, EteläSavon ja EteläKarjalan piiri